The present invention is directed to a miniaturizable write/read head for an optical storage system using an optical storage medium. The head includes a controlled laser as a light source which has one resonator face formed by the storage medium and is arranged with respect to the storage medium so that light reflected from the storage medium will be fed back into the laser for controlling the intensity of the emitted laser beam.
In optical storages, as known, information is in digital form and is written into the storage medium in parallel tracks with a high intensity or, respectively, is read out with a low intensity, sharply focussed light rays, usually emitted by a laser. In read-out control signals for the positioning of the write/read head are also usually acquired from the reflected light beam in addition to the output data signals. In optical storages, what is thereby involved among other things are two types of control signals, a malfocus signal and a track error signal, which are employed for the readjustment of the vertical spacing of the write/read head given focus errors, or respectively, for horizontal fine positioning of the write/read head given a track deviation of the light spot focus on the surface of the storage medium.
A laser is usually employed as a light source for both operating modes of write and read. A conventional image optics in the write/read head which focus the emitted laser beam onto the surface of the storage medium then contains optical elements for gating or splitting the reflected laser beam out of the beam path. The deflected reflected laser beam is then collected by photodetectors in order to acquire the control and read-out signals, respectively.
However, an article by Finck et al "Ein Halbleiterlaser zum Auslesen von Information "Philips Technische Rundschau, 39th Edition, No. 4, 1980/81, pages 101-111 discloses that information in the reflected light is detected with the assistance of the laser itself. In this case, the reflected light is not coupled out via a beam splitter, but is again imaged onto the exit mirror of the laser by the imaging optics. The active medium of the laser then reacts to the fluctuations of the light reflected from the surface of the storage medium with the reaction causing a modification of the emitted power. This modification is identified with the assistance of a photodetector.
In this known arrangement, the feedback effect is essentially defined by the collaboration of the actual laser resonator with an external resonator which is composed of the exit mirror of the laser, of an imaging optics and of the surface of the storage medium acting as a reflector. The relationship between the output power of the laser with and without feedback is predominantly defined by the reflective properties of the storage medium and of the image optics as well.
The feedback effect is an event that is relatively physically complicated. However, it is initially obvious that the laser effectively exhibits fewer radiation losses due to the feedback radiation and therefore oscillates at a higher power level given the same current. Without discussing this in great detail, it should be pointed out that the external resonator also effect modifications of the laser mode in interaction with the actual laser resonator.
In this context, U.S. Pat. No. 3,941,945, whose disclosures are incorporated by reference and which claims priority from German application No. 22 44 119, discloses a further, optical sensor for image plates wherein the surface of the storage medium to be read is incorporated into the laser structure so that the radiation reflected or scattered by it influences the intensity of the light vibration. In order to thereby increase the part of the light directed back into the laser from the storage medium, the end face of the laser directed toward the storage medium can be fashioned as an optical lens and be coated with an antireflection coating. Thus, the focal point of this imaging system should lie in the plane of the surface of the storage medium. A semiconductor diode operating in a non-conducting direction and united with the actual semiconductor laser can thereby be used for detecting the light variation. However, the semiconductor laser diode can also be fed with constant current and the read-out signal can be acquired from the electrical alternating voltage at the laser diode influenced by the intensity of the light vibration.
An information reproduction system is disclosed in U.S. Pat. No. 4,005,259, which is based on the same Japanese application as German Pat. No. 25 05 795. This information disclosure system is also based on the exploitation of the feedback effect. In this sytem, a semiconductor laser is positioned with respect to the storage medium so that emitted light, after passing through the non-reflecting lateral surface facing the storage medium, selectively impinges reflecting or respectively non-reflecting regions of the storage medium and again enters through the non-reflecting lateral face of the laser after the reflection. The different reflectivity of the storage medium is to be matched to the resonance of the laser diode so that an on/off control of the emission of the laser light occurs dependent on the selective incidence of the emitted laser light onto the reflecting or respectively non-reflecting regions of the storage medium. In this known information reproduction system, at least one lens is provided between the exit face of the semiconductor laser and the surface of the storage medium and the lens is provided as imaging optics or respectively for focussing the emitted laser beam onto the surface of the storage medium.
The one thing which is shared by the known write/read head or respectively by the read heads for an optical storage system in the case of an exclusive read-out memories, is that the laser beam must definitely be focussed on the surface of the storage medium and the imaging optics are therefore arranged between the laser and the surface of the storage medium. Since the distance variations are unavoidable during operation, the assistance of a highly effective control loop guarantees that this focus condition is always observed by readjusting the vertical spacing of the write/read head with reference to the surface of the storage medium. This focus condition is critical in optical storage because, as stated, both read-out signals as well as control signals are derived from the differing light and the exact focussing has an essentially co-determining influence on the signal-to-noise ratio of these signals.